how do I get a copy of mountain lion to start upgrading my OS from LION 10.7.5

I currently have LION on my mid 2010 iMac and my late 2008 MacBook Pro. Several banking sites have stopped supporting certain Safari versions which leaves me unable to do some online banking via my safari browser (V 6.1.6).

I have tried everything via the App store to find a download of mountain lion so I can begin to update both the iMac and the MacBook pro but CANNOT find any downloads for mountain lion. this I cannot progress towards a later version of the Mac OSX OR a later version of Safari. I am becoming significantly frustrated with the mac App store and the significant LACK of any support info from MAC. Has anybody got any suggestions/advice that might be useful to me?